Insights into erythromycin action from studies of its activity as inducer of resistance

Appreciation of the inducibility of erythromycin resistance began as an observation in the clinical bacteriology laboratory during susceptibility testing of erythromycin-resistant clinical isolates of Staphylococcus aureus. It was noted that inhibition zones surrounding spiramycin, lincomycin, and pristinamycin I (streptogramin B family) test disks placed close to an erythromycin test disk deviated from the expected circular shape and assumed a distorted ‘‘D’’ shape instead. Such observations suggested a possible antagonistic interaction between erythromycin, on the one hand, and spiramycin, lincomycin, or pristinamycin I, on the other (5, 7). The interaction turned out to be a functional rather than a physical antagonism, and out of these observations grew the notion of erythromycin-inducible resistance toward erythromycin, initially (41, 55), and then, more generally, toward all the macrolide, lincosamide, and streptogramin type B (MLS) antibiotics (57). Vazquez (50) and Vazquez and Monro (52) showed that antibiotics belonging to each subclass of the MLS antibiotics competed with chloramphenicol for uptake by intact cells. Competition for binding to purified 50S ribosome subunits was shown only for macrolides and lincosamides but not for streptogramin type B antibiotics. Collectively, these observations suggested that an alteration of 50S subunit function was involved in resistant cells. In a study of the time and concentration dependence of induction, Weisblum et al. (58) showed that (i) the optimal erythromycin concentration for induction was between 10 and 100 ng/ml, the threshold of its inhibitory action, (ii) at the optimal inducing concentration of erythromycin cells became phenotypically resistant within 40 min, and (iii) ribosomes from induced cells apparently bound labeled erythromycin and lincomycin with a reduced affinity. By mixing ribosome preparations from susceptible and resistant cells and noting no loss of expected antibiotic binding activity, it was possible to exclude the alteration of the antibiotic by modifying enzymes present as contaminants in the ribosome preparation. Consistent with this picture, Allen (1) showed that cell extracts of resistant S. aureus carried out erythromycin-resistant protein synthesis in vitro, suggesting that a component of the protein-synthesizing machinery had been altered. A posttranscriptional methylation of a single adenine residue in 23S rRNA, comprising the induced biochemical alteration (30, 31), was located at Escherichia coli coordinate 2058 (A-2058) (47), and translational attenuation (16, 24), the mechanism for its regulation, was proposed. As discussed below, this unusual mechanism of gene regulation requires no repressor proteins but, instead, is based on the conformational isomerization of the ermC message to a translationally active form. How might this be achieved?